1. Field of the Invention
The present invention relates to a fastening arrangement for a split casing. More specifically, the present invention relates to a fastening arrangement which is especially advantageous, for example, when used for forming casings of hydraulic machines, such as turbines and compressors, as flangeless casings.
2. Description of the Related Art
A horizontally split type casing construction is commonly used for casings of turbines and compressors. In the horizontally split type casing construction, a casing is divided into two segments by a plane including a center axis thereof. Usually, each casing segment (casing half) is provided with plate-like flanges having a relatively large thickness at joint portions of the casing. The casing halves are coupled to each other by joining and fastening the flanges together by fastening bolts.
FIG. 7 illustrates a fastening arrangement of a conventional horizontally split type casing having flanges. FIG. 7 shows a section perpendicular to an axis of the casing.
In FIG. 7, reference numeral 100 designates a casing consisting of two casing halves 110a and 120a. 110b and 120b designate flanges formed at the joint portions of the casing halves 110a and 120a. The flanges 110b and 120b are fastened together by a plurality of fastening bolts 115. Each of the fastening bolts is provided with a bolt head 115d at one end thereof and a screw thread 115a at the other end thereof. Threaded bolt holes 120c which engage the screw threads 115a of the bolts 115 are provided on the flange 120b of one of the casing halves 120a. Further, the flange 110b of the other casing half 110a is provided with bolt holes 110c. In order to joint two casing halves 110a and 120a, the fastening bolts 115 are inserted into the bolt holes 110c of the flanges 110b of the casing half 110a and the threads 115a of the bolt 115 are screwed into the threaded bolt holes 120c on the flanges 120b of the casing half 120a until the bolt heads 115d are pressed against the upper face of the flange 110b. By tightening the fastening bolt 115, the flanges 110b and 120b are firmly pressed against each other by the bolt heads 115d and the screw threads 115a of the bolt 115. In this condition, the tensile force is generated on the shaft of the bolt by tightening the bolt 115. The reaction force of the shaft tensile force is exerted on the upper face of the flange 110b through the bolt heads 115d and also on the screw threads of the threaded bolt holes 120c in the opposite direction. Due to these reaction forces, the flanges 110b and 120b are pressed against each other.
However, in some cases, problems occur when the fastening arrangement using the flanges as illustrated in FIG. 7 is used for the horizontally split type casings of hydraulic machines such as turbines or compressors.
In hydraulic machines having rotors, the casings containing rotors must have strictly circular cross sections especially at the inner peripheries. However, in the turbines and compressors, since the temperature of the fluid passing through the casings is high, the temperature of the respective portions of the casings becomes high. If the temperature of the casing wall varies in the respective portions, a large thermal stress is generated by the difference in the amount of the thermal expansion of the respective portions of the casing. When a large thermal stress is generated, the casing tends to deform and concentricity of the cross section of the casing cannot be maintained. Further, in the turbines and compressors, the temperature of the fluid passing through the machines changes considerably due to a change in the operating load. In this case, if the casing is provided with flanges having a thickness larger than other portions of the casing, the change in the temperature of the flanges is late compared with the other portions. This causes a large temperature difference between the flanges and other casing portions when the rate of the change in the temperature of the fluid in the casing is high. Therefore, if the casing is provided with flanges having a large thickness, distortion of the casing may occur when the temperature of the fluid changes.
In turbines and compressors, rotors rotating at high speed are accommodated in the casings. Therefore, if distortion of the casing occurs, the outer periphery of the rotor (such as the tips of the turbine blades) contacts with the inner periphery of the casing. This may cause damage to the machine. It is true that the contact between the rotor and the casing can be avoided even in this case if the clearance between the tips of the turbine blades and the inner periphery of the casing is set at a relatively large value. However, in the hydraulic machines such as turbines and compressors, since the efficiency of the machine decreases as the tip clearance becomes larger, it is not practical to set the tip clearance to a large value.
In order to solve the problems explained above, a flangeless casing which eliminates the use of the flanges is used in some cases for the horizontal split type casing. The flangeless casing is a casing which does not use flanges such as shown in FIG. 7 for joining the casing halves. FIG. 8 is a sectional view similar to FIG. 7 which illustrates a typical fastening arrangement of a flangeless casing.
As can be seen from FIG. 8, the casing halves 210a and 220a of the flangeless casing 200 have semi-circular cross sections without flanges at joining faces. The bolt holes 210c and 220c for fastening bolts 215 are drilled in the tangential direction in the walls of the casing halves 210a and 220a. Spot facings are formed on the upper ends of the bolt holes 210c in order to obtain a close contact between the surfaces of the casing half 210a around the bolt holes 210c and the bolt heads 215d of the fastening bolts 215.
In the flangeless casings as shown in FIG. 8, since the flanges having a large thickness are not used, a non-uniformity of the thickness of the casing is smaller compared with the flanged casings and the distortion of the casing due to the change in the temperature of the fluid also becomes smaller. However, even in the flangeless casing in FIG. 8, the problems similar to the flanged casings may occur when the pressure or the temperature of the internal fluid is high.
In the flangeless casings in FIG. 8, since the spot facings 210d are provided, the diameters of the bolt holes 210c above the spot facings are required to be the same as the diameters of the spot facings 210d. Therefore, the diameter of the bolt holes 210c becomes much larger than the minimum diameter required for allowing the bolt 215 to pass through. This means that a larger amount of metal must be removed from the walls of the casing half 210a and reduced wall thickness portions are formed by the bolt holes 210c. As illustrated in FIG. 8 the wall thickness becomes the smallest (T1 in FIG. 8) at the portion where the spot facings are formed.
FIG. 9 schematically shows a section of the wall of the casing half 210a around the spot facings 210d taken along the line A—A in FIG. 8. As can be seen from FIG. 9, the wall is cut off in a cylindrical shape around the spot facings 210d and only a solid metal in the shape of the hatched area remains. The average wall thickness of the portions shown by the hatched area is represented by T2 in FIG. 9. In other words, the effective wall thickness of the casing around the spot facings is reduced to a substantially small value T2 when the spot facings are formed. Therefore, in the flangeless casing in FIG. 8, reduced wall thickness portions are formed in the casing 210a by the spot facings 210d. Since the distortion of the casing occurs at these reduced wall thickness portions when the internal pressure or temperature of the casing is high, problems similar to those of the flanged casing of FIG. 7 occur.
In order to prevent the formation of the reduced wall thickness portions, it is necessary to reduce the diameters d (FIG. 8) of the spot facings 210d. However, practically it is not possible to reduce the diameter of the spot facing beyond some limit for the reasons explained below.
During the operation of the hydraulic machines, a large shaft tensile force are required for the fastening bolts in order to hold the casing halves together against the force generated by the internal fluid pressure and thermal stress exerting on the casing halves in a direction separating the casing halves from each other.
This means that a large force must be transferred from the fastening bolts to the casing 210a through the contacts between the bolt heads 215d and the spot facings 210d. Generally, the fastening bolts are made of a material having a high strength such as a high tension alloy. On the other hand, usually, a material having a high strength are not used for the casing halves in order to facilitate machining of the casing (such as cutting and, if required, welding) and to obtain a high resistance to a low frequency fatigue. Therefore, the maximum value of the contact pressure between the bolt heads and the casing is limited by the material used for the casing. Consequently, the diameter of the spot facing must be sufficiently large in order to reduce the contact pressure between the bolt head and the casing to within the allowable limit determined by the material of the casing while maintaining a sufficiently large shaft tensile force of the fastening bolt. Thus, it is practically difficult to reduce the diameter of the spot facings in order to increase the effective wall thickness of the casing around the spot facings.
Further, it is preferable to dispose the fastening bolts at a smaller interval around the casing in order to obtain a large and uniform tightening force around the casing. However, since a large diameter is required for the spot facings, it is difficult to reduce the distance between the bolt holes 210c. A sufficiently large and uniform tightening force of the casing halves cannot be obtained in some cases in the flangeless casing in FIG. 8.